As the switching of semiconductor switches such as, for example, IGBT (Insulated Gate Bipolar Transistor) modules becomes faster and faster and currents become higher and higher, it also becomes more difficult to limit turn-off voltages to a permissible range. When the permissible voltage across the controlled path of a semiconductor switch is exceeded, it will be destroyed which should be prevented under all circumstances. A particularly critical case is that of a short circuit since then the rate of current change di/dt is greater. The rate of current change di/dt induces in the stray inductances a voltage which is added to the voltage present in any case. During this process, the permissible voltages can be easily exceeded, particularly across the controlled path of the semiconductor switch.
One approach which is simple in theory consists in reducing the stray inductances. In practice, however, this can be achieved only with difficulty or not at all. One method frequently used in practice consists in using zener diodes in protective circuits, which is known as active clamping. This method consists in the semiconductor switch being slowed down in the turn-off process. Such an arrangement is described, for example, in DE 44 28 675 Al and comprises one or more zener diodes which are connected, for example, between the gate and the collector of an IGBT. If this is used many times, however, this arrangement leads to high power dissipation in the zener diodes and to thermal problems.